1. Field of the Invention
The present invention relates to a thermosetting powder paint composition, and more particularly, a thermosetting powder paint composition which can form a baked coating film having excellent external characteristics (eg. smoothness), physical properties (eg. mar resistance and recoating property), weather resistance, ultraviolet ray resistance, and chemical properties (eg. acid rain resistance and solvent resistance), and has excellent storage stability.
2. Description of the Related Art
Powder paints have a lot of merits such as low crisis of fire and poison and high workability, as compared with a solvent paint, and application field thereof has been recently enlarged. Among them, a thermosetting acrylic resin is regarded as promising from the viewpoints of gloss, weather resistance, chemical resistance and mechanical strength, and various investigations have been made for developing a thermosetting acrylic resin for a powder paint. As a conventional powder paint composition containing a thermosetting acrylic resin, there is generally used a composition prepared by compounding a thermosetting agent into an acrylic resin having a molecular weight of 1,000 to 30,000 which is a copolymer of an alkyl ester of acrylic acid or methacrylic acid with a glycidyl ester of acrylic acid or methacrylic acid (for example, DE-A Nos. 2,057,577 and 2,014,914). However, a coating film formed from this composition does not have satisfactory smoothness,,strength, chemical resistance and metal adherence.
There are also suggested a composition which provides a coating film having improved smoothness, prepared by compounding a cellulose derivative into a composition composed of an acrylic resin and a curing agent (for example, Japanese Patent Application Laid-Open (JP-A) No. 48-7,943), and a composition which provides a coating film having improved chemical resistance and adherence, prepared by compounding a polyvalent epoxy resin into a composition composed of an acrylic resin and a curing agent (for example, JP-A No. 48-17,844). However, coating films formed from powder compositions composed of these compositions do not satisfy in the point of strength.
Regarding the solvent paint, for example, JP-A No. 61-151,272 discloses a paint having high weather resistance which contains as a main resin component a graft polymer prepared by radical copolymerixzation of (A) a radical polymerizable compound mainly composed of a (meth)acrylate, (B) a radical-polymerizable silicone macro monomer, and (C) a cross-linkable monomer having a non-radical-polymerizable reactive functional group. However, since the silicone macro monomer of this paint is used in the graft polymer in high concentration (10 to 30% by weight), a coating film formed from this paint is poor in clear brilliance and recoating property while it is excellent in weather resistance and solvent resistance. Further, this paint having high weather resistance is a solvent type paint as is apparent from the formulation of a resin, curing agent and the like and simultaneously from the descriptions of examples. A solvent paint is not preferable in the viewpoints of atmosphere protection and resources saving as described below.
Recently, there are serious environmental problems such as air pollution, earth warming, acid rain and the like. Particularly, in the technical field of a paint, there occurs an air pollution problem when organic solvents generated in large amount in a painting process or a baking process are discharged into atmosphere, viewed from the standpoint of producers. On the other hand, there occurs a problem of the durability of a coating film when painted products are exposed to weather circumstances (eg. rain, snow, mist and smog), soil, sewage and the like which has been remarkably acidified, viewed from the standpoint of consumers. Further, when a solvent paint is used, an apparatus for closing system preventing leakage of organic solvents into atmosphere is required, therefore, the solvent paint is disadvantageous as compared with a powder paint which does not require such a large apparatus. Therefore, it is very effective to use a powder paint rather than a solvent paint from view points of environmental protection and resources saving, and transition from a solvent paint to a powder paint is tendency of the times.
However, in spite of increased demand for a powder paint due to such background, known powder paint technologies are problematical since a coating film formed by thermal setting is inferior in external characteristics (eg. smoothness and clear brilliance), physical properties (eg. impact resistance, adherence, mar resistance and re-coating property), weather resistance, ultraviolet ray resistance, and chemical properties (eg. acid rain resistance and solvent resistance), as compared with those in solvent paint technologies. Among these problems, acid rain resistance, solvent resistance and mar resistance are particularly problematical.
Regarding the powder paint technology, there is known a technology in which a thermosetting powder paint containing as a resin component an acrylic polymer having a glycidyl group and a dicarboxylic acid or polyvalent carboxylic anhydride as a curing agent is coated on a substrate (grounding), then cured thermally to form a coating film on the substrate (grounding), as disclosed, for example, also in U.S. Pat. No. 3,845,016 (Labana et at.) and U.S. Pat. No. 3,919,347 and JP-A No. 5-112,743, and the like. However, a cured coating film obtained by these technologies is inferior particularly in acid rain resistance, solvent resistance and mar resistance though it is excellent in external characteristics (smoothness).
For the purpose of solving the above-described problems of the known technologies, there has been made progress in investigation and development of another improving technology in which the surface of a coating film is modified by introducing a silicone compound having reactive functional groups such as a silanol group and alkoxysilyl group into a resin component of a powder paint composition, as disclosed, for example, in U.S. Pat. No. 4,877,837 (Reisin et al.). However, this powder paint composition is remarkably inferior in storage stability of a paint since a silanol group and an alkoxysilyl group having high reactivity exist in the silicone compound.
Further, Japanese Patent Application Publication (JP-B) No. 3-43,311 discloses a powder paint composition composed of a vinyl co-polymer of a glycidyl group-containing vinyl monomer, a silicon-containing vinyl-based monomer and the other monomers, and an aliphatic divalent acid. The silicon-containing vinyl monomer used in this composition is a compound which has a functional group (eg. an alkoxysilyl group) having high reactivity in addition to a vinyl-polymerizable unsaturated double bond, and has only one silicon atom in one molecule. A coating film obtained from this powder paint composition gives poor storage stability of a paint and poor acid rain resistance and solvent resistance of the coating film while gives excellent smoothness of the coating film, since the silicon-containing vinyl monomer has a functional group having high reactivity other than a unsaturated double bond, and the content of silicon per one molecule is low.
JP-A No. 09-505,847 discloses a technology regarding a powder paint suitable for painting a car body, wherein a specific epoxide group-containing polyacrylate resin is used as a binder. This powder paint is formed by using as a binder an epoxide group-containing polyacrylate resin obtained by polymerizing the following components (a) to (d).
(a) 10 to 50% by weight of an ethylene-based unsaturated monomer containing at least one epoxide group in the molecule or a mixture thereof, PA0 (b) 5 to 84.99% by weight of an aliphatic or alicyclic ester of (meth)acrylic acid or a mixture thereof, PA0 (c) 0.01 to 4.99% by weight of an ethylene-based unsaturated compound having statistically at least one polymerizable ethylene-based unsaturated group and at least two structural units represented by the general formula (--SiR.sup.1 R.sup.2 --O--) [R.sup.1 and R.sup.2 are the same or different and represents a C1 to C8 aliphatic or alicyclic group or a optionally substituted phenyl group] per one molecule, and PA0 (d) 5 to 84.99% by weight of an ethylene-based unsaturated monomer other than a, b and c, or a mixture thereof. PA0 (1) a based paint colored is applied on the surface of a substrate, PA0 (2) a polymer coating film is formed from the paint applied in the step (1), PA0 (3) a transparent powder paint containing as a binder en epoxide group-containing polyacrylate resin is applied on the base coating film thus obtained, and subsequently PA0 (4) the base coating film is baked together with the transparent powder paint, PA0 (1) a radical-polymerizable polysiloxane or an organopolysiloxane having a SH group in the molecule is selected as the organopolysiloxane, and the copolymer in the form of a graft or block is used. PA0 (2) The amount used of the organopolysiloxane used is at least 5% by weight in the copolymer. PA0 (i) When a silicone compound is used in large amount in producing a paint composition or a resin component, the transparency and clear brilliance of a coating film in the case of clear coat, and the adherence of the coating film with an under coating film and the re-coating property between top coating films are poor. An apparatus used for producing a paint composition and a resin composition is remarkably stained with a large amount of a silicone compound. Furthermore, the silicone compound which is remained in the apparatus gives bad effects to the other paint compositions and resin components, especially, it may reduce the appearance of the paint film. PA0 (ii) A coating film tends to be repelled of rejected on a substrate (grounding) in producing the coating film, to form an incomplete coating. PA0 (iii) It is difficult to manifest simultaneously excellent properties regarding the properties of a coating film such as external characteristics, physical properties, weather resistance, ultraviolet ray resistance and chemical properties. PA0 (iv) The storage stability of a paint composition is poor. PA0 (i) Compatibility in preparing a paint composition is improved, and pollution in production is prevented, PA0 (ii) The complete coating ability of a coating film on a substrate (grounding) is improved, PA0 (iii) Excellent properties are simultaneously manifested regarding the properties of a coating film such as external characteristics (eg. smoothness and clear brilliance), physical properties (eg. mar resistance and recoating property), weather resistance, ultraviolet ray resistance, and chemical properties (eg. acid rain resistance and solvent resistance) of a coating film formed by thermosetting, and particularly, acid rains resistance, recoating property, solvent resistance and mar resistance are remarkably improved, and PA0 (iv) The storage stability of a paint composition is improved. PA0 (1) The organic modified polysiloxane (B) is previously dissolved in a monomer component, and bulk polymerization or solution polymerization is conducted. In the case of a solution polymerization, an organic solvent is removed after the polymerization; PA0 (2) After conducting the solution polymerization using an organic solvent, the organic modified polysiloxane (B) is added and uniformly dispersed in the solution, then, the organic solvent is removed; and the like. For example, in adding the organic modified polysiloxane (B) having a reactive functional group which can react with an epoxy group carried by the acrylic copolymer (A), it is necessary to appropriately select the reactive functional group, the amount of the functional group, the molecular weight and the like of the organic modified polysiloxane (B) so as not to generate a gelled substance which is not preferable for the production process. PA0 R.sup.1 and R.sup.2 represent a direct bond, alkylene group having 1 to 8 carbon atoms, phenylene group or derivative thereof, R.sup.3 to R.sup.8 represent an alkyl group having 1 to 8 carbon atoms, phenyl group or derivative thereof, and n1 represents a number from 1 to 1,500, ##STR3## wherein the formula (II), X.sup.3 is a group represented by any of the above-described general formulae (a) to (e), R.sup.9 to R.sup.15 represent an alkyl group having 1 to 8 carbon atoms, phenyl group or derivative thereof, R.sup.16 represents a direct bond, alkylene group having 1 to 8 carbon atoms, phenylene group, alkylene group derivative, phenylene group derivative, or hydrocarbon group having an ether bond and/or ester bond, and n2 represents a number from 1 to 1,500, ##STR4## wherein the formula (III), X.sup.4 is a group represented by any of the above-described general formulae (a) to (e), R.sup.17 to R.sup.25 represent an alkyl group having 1 to 8 carbon atoms, phenyl group or derivative thereof, R.sup.26 represents a direct bond, alkylene group having 1 to 8 carbon atoms, phenylene group, alkylene group derivative, phenylene group derivative, or hydrocarbon group having an ether bond and/or ester bond, x1 represent a number from 1 to 1,300, and y1 represents a number from 1 to 200, ##STR5## wherein the formula (IV), X.sup.5, X.sup.6 and X.sup.7 are a group represented by any of the above-described general formulae (a) to (e), R.sup.30 to R.sup.36 represent an alkyl group having 1 to 8 carbon atoms, phenyl group or derivative thereof, R.sup.27 to R.sup.29 represent a direct bond, alkylene group having 1 to 8 carbon atoms, phenylene group, alkylene group derivative, phenylene group derivative, or hydrocarbon group having an ether bond and/or ester bond, x2 represent a number from 1 to 1,300, and y2 represents a number from 1 to 200.
In the technology regarding a powder paint disclosed in JP-A NO. 09-505,847, it is a characteristic that an epoxide group-containing polyacrylate resin modified by copolymerizing an ethylenically unsaturated polysiloxane macro monomer with other vinyl monomer is produced as a resin-forming component constituting a powder paint composition.
In this technology, it is essential to use an ethylenically unsaturated polysiloxane macro monomer, and there is a disclosure that it is used when
for producing a two-layer coating film on the surface of a substrate using this powder paint. In this technology, though adherence between the base coat and the top coat when the powder paint is baked on onto the base coating film is excellent, re-coating property between top (clear) coats, namely relative adherence obtained when the powder paint is painted and baked, and the powder paint is further painted and baked on the resulted baked coating film is not necessarily excellent.
JP-B No. 3-45,111 discloses a thermosetting resin composition containing as essential components (A) 100 parts by weight of a thermosetting resin and (B) 0.01 to 10 parts by weight of a glycidyl group-containing silicone-based additive having a number average molecular weight of 150 or more having no vinyl-polymerizable unsaturated bond.
JP-B No. 4-70,331 discloses a method for producing a thermosetting resin for a powder paint, modified by a silicon-containing compound, wherein a thermosetting resin selected from a polyester, epoxy resin and vinyl polymer or a resin-forming component thereof is allowed to react with the silicon compound having no vinyl-polymerizable unsaturated bond in an amount from 0.01 to 30% based on the weight of the resin or resin-forming component.
In the technologies disclosed in JP-B Nos. 3-45,111 and 4-70,331, the storage stability (solid-phase reaction resistance) of the paint has been poor since a silane-based compound having high re-activity is used in the molecule of the silicon-containing compound though the appearance (smoothness) of the coating film is excellent, and improvement in acid resistance and mar resistance of the coating film has been insufficient since the content of silicon occupying one molecule is low.
JP-A 9-241,538 discloses a powder paint containing a silicone/acryl-graft copolymer as a main component in which an organopolysiloxane and an acrylic polymer are bonded in the form of a graft or block, wherein the side chain of the acrylic polymer in the co-polymer contains one or more groups selected from an epoxy group, hydroxyl group and carboxyl group, and the glass transition temperature is 45.degree. C. or more.
This technology discloses the following points.
However, if an organopolysiloxane compound is used in the amount used as disclosed in this technology, the transparency and clear brilliance of a coating film in the case of clear coat becomes low, and the adherence of the coating film with a base-coat film and re-coating property between the top coats are poor.
EP-A No. 275,051 discloses a powder paint containing a polyacrylate resin produced by polymerizing as a binder an ethylene-based unsaturated monomer in the presence of a silicone resin. However, a large amount of the silicone resin herein described causes decrease in transparency and clear brilliance in the case of clear coat, decrease in adherence with a coating film and re-coating property between top coats.
When the above-described various problems are summarized, the conventional technologies regarding thermosetting powder paint compositions containing a silicone compound had any of the following problems (i) to (iv).